Believe it or not I have a purple tang, yellow tang, gem tang, long nose black tang, sailfin tang, blue hippo tang, and a yellow eye kole tang all in the same 750XXL.
I’m not saying I recommend it but I did a lot of research and had a game plan for this before I even got the tank.
These were among the first fish added to the tank and with the exception of the long nose black tang, all were tiny when I got them, as small as I could find (some no bigger than a quarter). Then they were all QT’d and added to the new tank around the same time.
Most are what I would call a medium size tang now (a little larger than palm size) and get along great.
The black tang was added later but as a larger specimen it hasn’t paid much attention to the smaller tangs and they haven’t shown any aggression towards it.
Another thing I did to try to keep aggression to a minimum is to feed very heavy, a large variety of foods with nori available daily.
Of course this requires a filtration system designed for heavy-in-heavy-out nutrient management. Again though, this was all planned from the start even before I bought the tank.
When I first added them into the tank there was some bickering while establishing a pecking order. Any time I saw this i’d shut the lights out for a few hours so when they came back on it was a new day in their minds.
My tangs will flash at each other now and then now but no real aggression, I never see any ripped fins or anything like that. Shoot, they even school together and swim around the tank sometimes in the evening.
All that being said I wouldn’t add another tang now that they’re established in the tank, I expect they’d probably kill it if I did.
So it can be done, but do your research, make a game plan, and have a backup plan if things aren’t working out (such as taking them back to your LFS even if it means giving away an expensive fish that you’ve put through QT).